You're making the assumption that being able to perform well in tougher classes at school means that you are smart.

People drop out from technical studies into humanities because they did not perform well academically in their technical studies. Just because a group of people can perform better at school than another group of people are not smarter. One group is better at school. And school is not an indicator as smartness. Just because a class is harder does not mean that the people who do well in it are smarter. Performance at school is not a direct indicator of smartness.